In conventional charged-particle-beam (CPB) projection-exposure methods and apparatus, such as methods and apparatus employing an electron beam for transferring a pattern defined on a reticle onto a sensitive substrate, specific techniques are employed for correcting certain imaging errors such as image rotation and suboptimal focus.
A typical technique is illustrated in FIG. 5, depicting an adjustment of the focal position of an image being projected onto a sensitive substrate. Specifically, an electron beam 1, carrying an image of a pattern defined on an upstream reticle (not shown), is refracted by a lens 30 so as to produce an image on a surface of a sensitive substrate 6 such as a semiconductor wafer. The lens 30 can be an electromagnetic lens or an electrostatic lens. Whenever the electron beam 1 does not converge to form a focused image on the substrate 6 (a condition indicated by the dotted line), the focal position of the electron beam 1 is axially changed by adjusting the lens 30. Such an adjustment is performed until a properly focused image is formed on the surface of the substrate 6 (as indicated by the solid line).
In an electron-optical system, when performing image adjustment using an electromagnetic lens 30, it is possible to change the focal position and rotation of an image by changing the electrical current flowing through the lens coil of the lens. Such a change in current causes a corresponding change in the lensing action of the electromagnetic lens, which causes a corresponding change in the imaging position.
Certain conventional CPB optical systems also employ, in association with primary electromagnetic and/or electrostatic lenses, auxiliary lenses that generate a relatively weak magnetic or electrical field (compared to the associated primary lenses). The energization of such auxiliary lenses is variable so as to achieve an adjustment of the corresponding primary lens as required.
Further with respect to conventional CPB projection-exposure apparatus, the position of the image can be shifted in a direction parallel to the surface of the substrate by subjecting the charged particle beam to a magnetic or electrical field extending in a direction perpendicular to the field of the corresponding primary lens. Such action is termed "deflection," and an appliance that performs deflection is termed a "deflector." Unfortunately, whenever a charged particle beam is deflected, accompanying deflection aberrations are typically generated. Deflection aberrations are manifest as one or more of, for example, focal shift, rotation, magnification change, astigmatism, and astigmatism distortion, etc., in the image as formed on the substrate. Such aberrations are undesirable because they degrade the resolution and distort the shape of the image. To correct or at least reduce such aberrations, deflection-aberration-correction lenses are typically utilized, thereby further adding to the complexity of the CPB optical system.
In microlithographic projection-exposure apparatus that employ light for forming the image of the reticle pattern on the substrate surface, corrections of rotation and focal position of the image are generally performed using a correction mechanism that interacts with the substrate stage. I.e., the desired exposure location on the substrate is aligned with the reticle image by changing the angular orientation of the substrate about the optical axis of the apparatus and/or by changing the "height" of the substrate (i.e., position of the substrate along the optical axis). Such changes are typically performed using a stage-rotation-adjustment mechanism and a stage-height-adjustment mechanism, respectively.
Stage shifting is illustrated in FIG. 6. Ideally, the electron beam 1 forms an image on the substrate 6 (solid line) for best image focus. However, if the substrate is at a position indicated by the dashed line, proper focus of the image on the sensitive substrate 6 can be achieved by moving the substrate stage (holding the sensitive substrate 6) downward as indicated by the arrow. The advantage of such stage-shifting is that residual deflection aberrations (otherwise generated when rotation correction and focus correction are made using a corrective deflector or the like) are not produced.
During exposure performed using certain types of conventional CPB projection-exposure apparatus, the reticle is moved by a reticle stage in synchrony with motion of the substrate. With such apparatus, it is necessary not only to correct rotation and "height" errors of the substrate, but also to correct rotation and height errors of the reticle. Such corrections are typically made by concerted actions of an image-correction lens, a reticle-stage-correction mechanism, and a substrate-stage-correction mechanism.
Since a stage has a relatively large mass, its resonance frequency is low. Consequently, movement imparted to a stage for the purpose of making a correction as noted above is limited, i.e., rotation, lateral displacement, and height corrections of the stage only can be made at a maximum frequency of several hundred Hz. In order to achieve high throughput, the projection-exposure apparatus must be able to perform exposures of reticle subfields at a rate of several KHz or higher. If corrections are needed from subfield to subfield, mechanical stage corrections simply cannot be made sufficiently rapidly to ensure proper correction from subfield to subfield and still achieve satisfactory throughput.
Whereas imaging corrections can be made more rapidly using an image-correcting lens, such corrections are limited to a narrow range of beam deflection. This is because large changes to the beam imparted by an image-correction lens introduce substantial aberrations that can be difficult to correct. In other words, an image-correcting lens can introduce image defocusing and distortion as the lens is being used to correct other imaging problems. Therefore, the range of operation of an image-correcting lens is typically very narrow.
In view of the foregoing, there is a need for improved methods and apparatus for performing imaging correction in CPB projection-exposure apparatus.